using tuple as value type for associative array

[Flaze07]

when I do
Tuple!(uint, "first", uint, "second")[string] what; //I tried 
aliasing the tuple as well
what["something"].first = 20;
I get range error
but when I do
uint[string] what2;
what2 = 20;
I get none of those range error, so...how do I use tuple as value 
type for associative array ?